OneWayGNSSRange.java
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* CS licenses this file to You under the Apache License, Version 2.0
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package org.orekit.estimation.measurements.gnss;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import org.hipparchus.analysis.differentiation.Gradient;
import org.orekit.estimation.measurements.AbstractMeasurement;
import org.orekit.estimation.measurements.EstimatedMeasurement;
import org.orekit.estimation.measurements.InterSatellitesRange;
import org.orekit.estimation.measurements.ObservableSatellite;
import org.orekit.propagation.SpacecraftState;
import org.orekit.time.AbsoluteDate;
import org.orekit.time.FieldAbsoluteDate;
import org.orekit.utils.Constants;
import org.orekit.utils.PVCoordinatesProvider;
import org.orekit.utils.ParameterDriver;
import org.orekit.utils.TimeStampedFieldPVCoordinates;
import org.orekit.utils.TimeStampedPVCoordinates;
/** One-way GNSS range measurement.
* <p>
* This class can be used in precise orbit determination applications
* for modeling a range measurement between a GNSS satellite (emitter)
* and a LEO satellite (receiver).
* <p>
* The one-way GNSS range measurement assumes knowledge of the orbit and
* the clock offset of the emitting GNSS satellite. For instance, it is
* possible to use a SP3 file or a GNSS navigation message to recover
* the satellite's orbit and clock.
* <p>
* This class is very similar to {@link InterSatellitesRange} measurement
* class. However, using the one-way GNSS range measurement, the orbit and clock
* of the emitting GNSS satellite are <b>NOT</b> estimated simultaneously with
* LEO satellite coordinates.
*
* @author Bryan Cazabonne
* @since 10.3
*/
public class OneWayGNSSRange extends AbstractMeasurement<OneWayGNSSRange> {
/** Emitting satellite. */
private final PVCoordinatesProvider remote;
/** Clock offset of the emitting satellite. */
private final double dtRemote;
/** Simple constructor.
* @param remote provider for GNSS satellite which simply emits the signal
* @param dtRemote clock offset of the GNSS satellite, in seconds
* @param date date of the measurement
* @param range observed value
* @param sigma theoretical standard deviation
* @param baseWeight base weight
* @param local satellite which receives the signal and perform the measurement
*/
public OneWayGNSSRange(final PVCoordinatesProvider remote,
final double dtRemote,
final AbsoluteDate date,
final double range, final double sigma,
final double baseWeight, final ObservableSatellite local) {
// Call super constructor
super(date, range, sigma, baseWeight, Collections.singletonList(local));
// The local satellite clock offset affects the measurement
addParameterDriver(local.getClockOffsetDriver());
// Initialise fields
this.dtRemote = dtRemote;
this.remote = remote;
}
/** {@inheritDoc} */
@Override
protected EstimatedMeasurement<OneWayGNSSRange> theoreticalEvaluation(final int iteration,
final int evaluation,
final SpacecraftState[] states) {
// Range derivatives are computed with respect to spacecraft state in inertial frame
// Parameters:
// - 0..2 - Position of the spacecraft in inertial frame
// - 3..5 - Velocity of the spacecraft in inertial frame
// - 6..n - measurements parameters (clock offset, etc)
int nbEstimatedParams = 6;
final Map<String, Integer> parameterIndices = new HashMap<>();
for (ParameterDriver measurementDriver : getParametersDrivers()) {
if (measurementDriver.isSelected()) {
parameterIndices.put(measurementDriver.getName(), nbEstimatedParams++);
}
}
// Coordinates of both satellites in local satellite frame
final SpacecraftState localState = states[0];
final TimeStampedFieldPVCoordinates<Gradient> pvaLocal = getCoordinates(localState, 0, nbEstimatedParams);
final TimeStampedPVCoordinates pvaRemote = remote.getPVCoordinates(getDate(), localState.getFrame());
// Downlink delay
final Gradient dtLocal = getSatellites().get(0).getClockOffsetDriver().getValue(nbEstimatedParams, parameterIndices);
final FieldAbsoluteDate<Gradient> arrivalDate = new FieldAbsoluteDate<>(getDate(), dtLocal.negate());
final TimeStampedFieldPVCoordinates<Gradient> s1Downlink = pvaLocal.shiftedBy(arrivalDate.durationFrom(pvaLocal.getDate()));
final Gradient tauD = signalTimeOfFlight(new TimeStampedFieldPVCoordinates<>(pvaRemote.getDate(), dtLocal.getField().getOne(), pvaRemote),
s1Downlink.getPosition(), arrivalDate);
// Transit state
final double delta = getDate().durationFrom(pvaRemote.getDate());
final Gradient deltaMTauD = tauD.negate().add(delta);
// Estimated measurement
final EstimatedMeasurement<OneWayGNSSRange> estimatedRange =
new EstimatedMeasurement<>(this, iteration, evaluation,
new SpacecraftState[] {
localState.shiftedBy(deltaMTauD.getValue())
}, new TimeStampedPVCoordinates[] {
pvaRemote.shiftedBy(delta - tauD.getValue()),
localState.shiftedBy(delta).getPVCoordinates()
});
// Range value
final Gradient range = tauD.add(dtLocal).subtract(dtRemote).multiply(Constants.SPEED_OF_LIGHT);
final double[] rangeDerivatives = range.getGradient();
// Set value and state derivatives of the estimated measurement
estimatedRange.setEstimatedValue(range.getValue());
estimatedRange.setStateDerivatives(0, Arrays.copyOfRange(rangeDerivatives, 0, 6));
// Set partial derivatives with respect to parameters
for (final ParameterDriver measurementDriver : getParametersDrivers()) {
final Integer index = parameterIndices.get(measurementDriver.getName());
if (index != null) {
estimatedRange.setParameterDerivatives(measurementDriver, rangeDerivatives[index]);
}
}
// Return the estimated measurement
return estimatedRange;
}
}